A heterotrimeric G-protein in vertebrate photoreceptor cells is called transducin (Tαβγ), whose γ-subunit is a mixture of two components, Tγ-1 and Tγ-2. Tγ-2 is S-farnesylated and partly carboxyl methylated at the C-terminal cysteine residue, whereas Tγ-1 lacks the modified cysteine residue. To elucidate the physiological significance of the double modifications in Tγ, we established a simple chromatographic procedure to isolate Tγ-1, methylated Tγ-2 and non-methylated Tγ-2 on a reversed phase column. Taking advantage of the high and reproducible yield of Tγ from the column, we analyzed the composition of Tγ subspecies in the Tα-Tβγ complex which did not bind with transducin-depleted rod outer segment membranes containing metarhodopsin II. The binding of Tα-Tβγ with the membranes was shown to require the S-farnesylated cysteine residue of Tγ, whose methylation further enhanced the binding. This synergistic effect was not evident when Tα was either absent or converted to the GTP-bound form which is known to dissociate from Tβγ. Thus we concluded that a formation of the ternary complex, Tα-Tβγ-metarhodopsin II, is enhanced by the farnesylation and methylation of Tγ. This suggests that the double modifications provide most efficient signal transduction in photoreceptor cells.
CITATION STYLE
Ohguro, H., Fukada, Y., Takao, T., Shimonishi, Y., Yoshizawa, T., & Akino, T. (1991). Carboxyl methylation and farnesylation of transducin gamma-subunit synergistically enhance its coupling with metarhodopsin II. The EMBO Journal, 10(12), 3669–3674. https://doi.org/10.1002/j.1460-2075.1991.tb04934.x
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